The present invention relates generally to microwave radar altimeters, and particularly to frequency modulated continuous wave radar altimeters.
Prior frequency modulated continuous wave (FM/CW) altimeters generally have been comprised of an antenna for radiating and receiving microwave energy, a transmitter for generating a frequency modulated carrier signal, a mixer coupled between the antenna and transmitter for generating a resulting signal from the combination of the transmitted and reflected microwave signals, and receiver circuitry for processing the resulting signal into a signal indicating the distance between the antenna and a target. The frequency of the carrier signal is changed linearly with time, and transmitted through the antenna to a target. When the signal reaches the target, a portion of it is reflected back to the antenna. By the time the signal is reflected back to the antenna, the transmitter has advanced the frequency of the carrier signal to some higher value. The reflected (lower frequency) signal and the presently transmitted (higher frequency) signal combine in the mixer to produce a resulting signal. This resulting signal is typically a function of the frequency difference between the reflected and transmitted signals. This frequency difference is a measure of the time needed for the signal to travel between the antenna and target, and thus may be related to distance travelled.
Part 15 of the Federal Communication Commission's regulations for motion detectors permits the use of transmitters with a carrier frequency of 10.525 gigahertz (Ghz) and a maximum frequency deviation of plus/minus 25 megahertz (Mhz). This deviation in frequency, or modulation, determines the resolution of altimeter. Generally, the greater the permissible frequency deviation, the greater will be the accuracy or resolution of the altimeter. Thus, the maximum permissible frequency deviation is an extremely important factor when an altimeter requires resolution between relatively small distances.
If a frequency excursion or deviation of 40 Mhz is employed, a single sinusoidal cycle will emanate from the mixer (i.e., the resulting signal for each sweep in modulation) when the distance between the antenna and target is approximately 12.3 feet. This distance corresponds to one half the wavelength for the frequency excursion of 40 Mhz. If the target is 15.38 feet from the antenna, the mixer will produce a sinusoid one and one quarter cycles in length for each sweep. Thus, as the distance is increased, a longer sinusoidal signal will result from the mixer. The logical technique for processing this signal into a digital indication of distance is to count zero crossings. Thus, every time the resulting signal from the mixer crosses zero, a signal would be produced which could be readily counted. In the case where a 40 Mhz excursion is employed, the resulting signal would cross zero approximately every 6 feet.
The above technique is considered to be the most logical because zero crossings are very sharply defined, and in a distance measuring device such sharply defined points would be necessary for accuracy. However, there is an inherent problem with this technique when resolution within a few feet is required. This problem is due in part to the fact that the starting point of the resulting signal from the mixer is unknown or uncertain. For instance, if the distance between the antenna and target is 15.38 feet, then a sinusoid 1.25 cycles in length will result from the mixer in all cases. However, the sinusoid need not start at 0 degrees, and may begin at 30 degrees, 52 degrees, 84 degrees, 90 degrees, and so on. The exact beginning and ending of the sinusoidal resulting signal is determined by the position of the antenna with respect to the target in terms of multiples of half wavelengths at the carrier frequency. The consequence of this uncertainty in the starting point is that the number of zero crossings is affected. For example, when the distance from the antenna to the target is between 6 and 12 feet, the number of zero crossings counted would be either 1 or 2 (assuming a 40 Mhz frequency modulation). Likewise, when the distance is between 12 and 18 feet, the count would be either 2 or 3, and so forth. Consequently, if the zero crossing detector produces a 2 count, the target could be anywhere between 6 and 18 feet from the antenna. When the distance from the antenna to the target is less than 6 feet, there is an additional problem. At this distance, the count should be either 0 or 1. However, because of the symmetry present when the mixer output is a sinusoid less then 0.5 cycles in length, a zero crossing detector would produce 2 counts.
In order to increase the close range accuracy of an altimeter, a different counting technique is employed according to the present invention. In particular, the inflection points on the sinusoidal signal resulting from the mixer are counted rather than zero crossing points. The inflection points occur when the slope of the sinusoidal waveform changes from plus to minus and from minus to plus. When an inflection point counting technique is employed, the count will always be either zero or one when the distance being measured is within 6 feet. However, there is a difficulty with this approach, and that is that the inflection points are not sharply defined.
To solve this problem, the transmitter and receiver according to the present invention were designed to take advantage of the fact that the slope is zero at an inflection point. In particular, a double modulation technique is employed in the transmitter. The transmitter is generally comprised of a Gunn diode for generating a carrier signal, a varactor diode for modulating the frequency of the carrier signal, and a high frequency oscillator for impressing a tone upon the sawtooth voltage waveform input to the varactor diode. Thus, the high frequency oscillator modulates the means modulating the carrier signal. With this arrangement, the resulting signal from the mixer becomes effectively two identical sinusoidal waveforms displaced slightly in time by the high frequency tone. However, at the inflection points the sinusoids meet, because there is no modulation at the zero slope. With the receiver according to the present invention tuned to amplify only the high frequency tone, the output of the amplifier will be zero at each inflection point. This then creates a sharply defined point which may be readily counted.